Method of manufacturing chemical pulp or cellulose



Patented a... 27, 1931 UNITED STATES DONALD E. CABLE, 01F BROOKLYN, NEW YORK, ASSIGNOR TO OXFORD PAPER COM- or fibre, extentand rapidity of penetration IPANY, OF NEW YORK, N. Y.,

A CORPORATION OF MAINE EETHOD OF MANUFACTURING CHEMICAL PULP 0R CELLULOSE 11 0 Drawing.

This invention relates to a method of making chemical pulp or cellulose by digesting wood chips or other suitable fibrous mateis that by the use of a sustained pressure, pref-.

erably of nitrogen (or other suitable inert or non-oxidizing gas) upon the surface of the liquor used for cooking the wood chips are increased, while at the same timeoxida; tion is decreased, as compared with methods now in use. The pressure of propellant gas is obtained by injecting it, for instance, into the top of a freshly charged and covered ,digester until the inside pressure has reached 20 to lOO'lbs. per square inch (gauge). This initial pressure, reached at-the startof the cook, is to be sustained while the cook continues in the usual way.

The new step of process to which I have referred may be utilized in connection with the familiar sulfite process, in which shipped wood is placedin a digester together with a solution of sulfur dioxide, including some alkaline earth bisulfite such as calcium bisulfite, magnesium bisulfite, or both tog'etl1er.-- Such solution will have approximately one per cent combined sulfur-dioxide (associated chemically with'lime to form the alkaline earth bisulfite) and 3% r cent of uncombined sulfur dioxide. bove this liquor and the chips will be a certain amount of air. According to' the old practice, the

cover of the digester isthen lowered and bolted in place, and the contents of the digester heated by injecting steam at the bottom.

In accordance with the present invention a pressure of nitrogen gas (or other suitable inert or non-oxidizing gas) is introduced into the digester at thefbeginning of the cook. It is known that thevhigher the average gas or, digester pressure during the impregnation periodlbefore any part of the chips have been heated above the critical temperature of 110 (3.), the shorter will be the time fall.

Application filed October 27, 1927. Serial No. 229,279.

PATENT OFFICE required to reach maximum penetration.

The use of nitrogen gas, because of its rela tive insolubility in-water (under elevated pressures) permits the attainment of as high a pressure as desired within. a freshly charged digesterin a brief interval of time.

The digester pressure may be brought imme- 4 diately to 7 5 pounds per square inch (gauge) or more if desired, and once the desired pressure is attained there is no tendencyffor it to In fact, it continues to rise after the 1n ect1on of nitrogen gas into'the digester has been stopped. This further rise in pressure is caused by the increased partial pressure of the sulfur dioxide in the gasor vaporv portion of the digester, resulting from the radual liberation of sulfur dioxide gas rom the sulfite cooking acid during the period of rising temperature. .Preferably after the penetration-promoting gas has been introduced into .the digester, the com-. bined gas pressure should at no time before square inch (gauge), and in fact it should 1 the endof the cook be less than 20 lbs. per

any part or all of the. cook, than is normally 7 obta ned in the customary way ofconduct ing such cooks; nor should the ratio of oxygen to nitrogen with-inthe digester be great-- er than-1 to 6 by volume. 1 a

Optimum pressure conditions, which" by old methods are not attained untilafter the cookhas proceeded for aconsiderable time, are by my process established within the digester at a very much earlierstage, and even at the very beginning, of the cook, and .maintained. v

Important practical advantages" result from the use of the new process. In the first place, it is found that the use of the propellant non-oxidizing gas markedly decreases the bleach consumption of the resulting pulp. This is due in part at least to the fact that nitrogen (if nitrogen be the gas) is an inert gas which dissolves but little in the hot cooking liquor, or even at the beginning in by the gas,.the usual oxidation is diminished.

I On this point it has been found in a series of small scale experiments that if digester runs which have been cooked with an initial pressure of nitrogen gas of approximately 7.5 pounds (gauge) before the heating up began, be taken as standard, then (1) the cooking in the ordinary way (that is, without booster gas) required 67% more of bleaching material; (2) cooking with compressed air required 134% more bleaching material;

(3) cooking with compressed carbon-dioxide, 64% more bleaching material;'and (4) cooking with compressed sulfur dioxide, 24% more bleaching material.

Another advantage which the use of nitro gen exhibits in comparison with sulfurdioxide, and, to a less extent, in comparison with carbon dioxide (particularly at the be- .the non-oxidizing gas pressure sustained from the beginning of the cook is that the gas has a propellantfunction and increases the speed with which the cooking liquor penetrates the chips or fibres, whereby the chance of screenings formation is decreased. It is well known that screenings formation takes place to an extent inversely proportioned to. the degree of chip penetration at the time the charge reaches a temper-- ature of 110 C. Pulp screenings consti v tutes aworthless product, and is often formed at the sacrifice of wood'pulp when:

chip penetration at the critical stage of a cook is incomplete.

A further advantage resulting from the.

use of this invention is that because the gas pressure is put on at the beginning of the cook it isv possible to shorten the digestion period and hence to effect economy as to fuel and overhead, andalso to increase plant pro 'duction;

It will be evident that the increased speed of penetration of the liquor into the chips,

. the inhibition of deleterious oxidation,-the maintenance of pressure conditions, and other results following upon the application of the gas pressure, are functions of eachother and interactively effective to produce important individual and combination advantages which will be obvious to those'skilled in the art. For example, shortening of the time necessary for the completion of the cook means .that costs are reduced, output increased, and quality of the final product improved. Furthermore, it is known that prolonged cooking or bleaching operations, be,- sides being expensive, are apt to decrease the cool yield and alfect adversely the quality of the re. I

Nitrogen has been mentioned as possibly the preferred non-oxidizing gas; but it is contemplated that other gases may be employed,

for instance, carbon dioxide, or mixtures of nitrogen and carbon dioxide in any ratio. Although carbon dioxide 'is about 56 times more soluble in water than nitrogen at room temperature, it is only about 1M5 as soluble in water as sulfur dioxide at that temperature. Trouble from falling digester pressure due to the gradual solution of the injected gas, while totally absent in the case of nitrogen,'is only slightly noticeable in the case of carbon dioxide. With sulfur'dioxide, however, the tendency. is very pronounced;

though of course this particular objection be overcome by continuing the injection of the heated sulfur dioxide gas into the top of the digester, so as to replace the gas condensed or dissolved in the unsaturated cooking acid.- For reasons of economy and general effectiveness, however, I prefer to use nitrogen or some other gas that is both nonoxidlz ng and relatively insoluble in the digestive agent at ordinary room temperatures and pressures.

Air as the penetration-booster is 'objectionable, as compared with nitrogen, for instance, because the oxygen.content thereof affects deleteriously the quality of the pulp and increases the bleaching expense. Carbon dioxide might be considered a substantially inert gas, but it is not so inert as nitrogen, and it is relatively expensive, and also somewhat soluble in'the cooking liquor.

The use of the sustained gas pressure for aiding; chip penetration at the start of the cook will undoubtedly be found to be most beneficial in the case of-chips that are airdried (having a moisture content of 10% or less), and least beneficial in the case of water saturated chips (havinga moisture content of 50% or more). Between these limits the i value of this treatment should be inversely proportional to the moisture content of the chips being cooked. If all the pores and cavities of a chipare initially filled with water accompanying the green or freshly cut wood, it would be physically impossible to force more liquid (in the form of cookingliquor) into the chip by the application of pressure, no matter how high that pressure. Freshly cut-wood, such as spruce or balsam fir will frequently be found to carry 50% or more of lbs moisture. Forthis reason, in many mills the wood is allowed to dry in the air, or season,

for periods of a year or more. This naturally leads to a reduction in the moisture content to 20 or 30%. It hasalso been proposed that wood chips be dried artificially by means of such agents as hot flue gas. Artificially dried chips or chips from seasoned wood will benefit greatly from the application of the nonsufficient purity by taking the present Waste gases available from the stack at the outlet of 'the'acid making plant where water and lime compounds are treated with the burner gas from the sulfur burners. The sulfur burns With the oxygen of the air to give sulfur dioxide, and this sulfur dioxide is absorbed by the lime compounds and Water to give the cooking acid, and there is a residue of nearly pure nitrogen. By running this through a compressor the nitrogen can be obtained at the desired pressure at almost no cost in the plant.

There are other sources of reasonably pure nitrogen available around a mill which will undoubtedly serve excellently in accomplishing the effect desired by this modified pressure treatment. For instance, scrubbed or just filtered flue gas would act nearly as well as pure nitrogen, even though it would contain to offcarbon dioxide. Such gas would be much superior to air o'naccount of its much lower oxygen content. I

' Although I have referred particularly in the foregoing description to. the use of the non-oxidizing gas treatment in' connection with the sulfite process, it is to be understood that other methods of producing pulp,"as for instance,.the soda or sulfate processes, "may also be improved by adding the step of introducing'a pressure of non-oxidizing gas at the beginning of'the coo addition of the cooking liquor to the digester,

In order to further reduce oxidation during the cook, I may produce a vacuum as preliminary to introduction of the booster gas.

In applying a vacuum to a sulphite digester previous to the injection of a non-oxidizing gas, it is desirable that it be done before the in order to avoid the loss of free sulfur dioxide from said liquor. This vacuum is next displaced by an atmosphere of nitrogen (or other inert gas) before running in the liquor.

The preliminary vacuum treatment is effective in eliminating all traces of oxygen accompanying the air inside the digester. The advantage of, doing this may prove of suflicient value to warrant the added expense involved.

sulfite process In the appended claims lg te process deis meant the familiar sul scribed on'page 1 of this specification where lime, magnesia, or other base is used as the base of the bisulfite constituent of the solution.

agent under an initial penetration-boosting in the digester at the pressure of non-oxidizing gas, whlch 1s a permanent gas at ordinary temperature and pressure and is not substantially absorbed in the digestive process. p i

2. The method of'producing pulp which comprises subjecting wood chips or the likematerial to the action of a suitable digestive agent under a penetration boosting pressure of a relatively nonsabsorbable non-oxidizing gas which is a permanent gas at ordinary temperature and pressure, said pressure being sustained until digestion is complete;

3. As a step in the manufacture of pulp by thesulfite process, introducing non-oxidizing gas under penetration-boosting pressure into the digester at the beginning of the cook.

4. As a step in'the manufacture ofpulp by the sulfite process, introducing non-oxidizing gas under penetration-boosting pressure into the digesterat the beginning'of the cook, and

maintaining the pressure of gas until pone-- tration is complete;

1 5. The method of producing: pulp which comprises subjecting Wood chips or the'like material to the action'of a suitable digestive agent under a penetration-boosting pressure of a non-oxidizing gas that is relatively insoluble in'said agent.

6. The methodof producing. pulp which comprises subjecting Wood chips or the like material to the actioiiiofa suitable digestive agent under a of relatively inert gas.

7. 'The'method of producing pulp 'whichl comprises subjecting wood chips or the like penetration-boosting pressure materialtothe.actionof a suitable digestive agent under a o nitrogen.

'8. The method of producing pulp whi'cli cimprises subjecting dry wood chips or the li e gestive' agent under sustained penetrationboosting pressure of relatively inert gas.

9. As a step in the manufacture of pulp by the sulfite'proce'ss introducing a non-oxidizing gas under suitable pressureinto the digester at the beginning of the cook, and maintaining the pressure until thecook is complete, thereby effecting substantially complete chip penetration prior to the stage of the cook Whenthe digester charge'reaches a temperature of C.

10. As a step in the manufacture of pulp by the sulfite process, the establishing of'optimum penetration-boosting conditions withby introducing into the digester under suitable pressure a non-oxidizing gas or. combination of gases, and maintaining the pressure until the cook is complete.

11. As a step in the manufacture of by. the sulfite process, artificially building up the pressure in the digester, at the beginning of the cook, without access of oxygen, to a point at which chip penetration is boosted,

penetration-boosting pressure 105 material to theaction of a suitable di- Ill beginning of the cook pulp and maintaining the boosting pressure at least to the stage of the cook at which a temperature of 110 C. is reached in the digester.

'12. The method of producing pulp which 5 comprises subjecting wood chips or the like material to the action of a suitable digestive agent, under penetration-boosting pressure of a relatively non-absorbable non-oxidizing gas, which is a permanent gas at ordinary temperature and pressure, and maintaining the pressure from the beginning of the cook until the end thereof.

13. The method set forth in claim 12, in which the gas is chosen for its relative insolubility in the digestive agent.

- 14. The method of producing pulp by the sulfite process, which comprises applying a vacuum tothe chip filled digester, then introducing a non-oxidizing gas at substantially atmospheric pressure, then introducing the digestive agent, then adding more non-oxidizing gas until a penetration-boosting pressure is reached, and then beginning and completing the cook while maintaining the pres-; 5 sure, said non oxidizing gas being a, permanent gas at ordinary temperature and pressure.

15. The method of producing pulp b the sulfite process which comprises the to low so ing steps; applyin fla vacuum to the filled dige ster, inserting t e digestive solution, and adding a non-oxidizing gas until a penetration-boosting pressure is reached.

16. The method of producing pulp which comprises subjecting wood chips or the like material to the action of a suitable digestive agent, under penetration-boosting pressure of a non-oxidizing gas introduced in the gaseous phase, and maintaining the pressure 40\ from the beginning of the cook until the end thereof.

In testimony whereof I afiix my signature.

'DONALD E. CABLE. 

